U.S. patent number 6,814,046 [Application Number 10/788,324] was granted by the patent office on 2004-11-09 for direct fuel injection engine.
This patent grant is currently assigned to Nissan Motor Co., Ltd.. Invention is credited to Koji Hiraya, Isamu Hotta, Akihiko Kakuho, Eiji Takahashi, Hirofumi Tsuchida.
United States Patent |
6,814,046 |
Hiraya , et al. |
November 9, 2004 |
Direct fuel injection engine
Abstract
A direct fuel injection engine basically comprises a combustion
chamber, a piston with a cavity, a fuel injection valve, a spark
plug and a control unit. The fuel injection valve is configured and
arranged to directly inject a fuel stream into the combustion
chamber in a substantially constant hollow circular cone shape in a
stratified combustion region. The control unit is configured to
ignite a first air-fuel mixture formed directly after the fuel
stream is injected and prior to a majority of the fuel stream
striking the cavity when the direct fuel injection engine is
operating in a low-load stratified combustion region, and to ignite
a second air-fuel mixture formed after a majority of the fuel
stream is guided by the cavity toward an upper portion of the
combustion chamber above the cavity when the direct fuel injection
engine is operating in a high-load stratified combustion
region.
Inventors: |
Hiraya; Koji (Yokohama,
JP), Takahashi; Eiji (Yokosuka, JP),
Tsuchida; Hirofumi (Yokosuka, JP), Hotta; Isamu
(Yokohama, JP), Kakuho; Akihiko (Yokohama,
JP) |
Assignee: |
Nissan Motor Co., Ltd.
(Yokohama, JP)
|
Family
ID: |
32964982 |
Appl.
No.: |
10/788,324 |
Filed: |
March 1, 2004 |
Foreign Application Priority Data
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Apr 25, 2003 [JP] |
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2003-121610 |
May 30, 2003 [JP] |
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2003-154056 |
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Current U.S.
Class: |
123/294; 123/276;
123/305; 123/304 |
Current CPC
Class: |
F02D
41/3023 (20130101); F02B 17/005 (20130101); F02B
23/101 (20130101); F02D 13/0215 (20130101); F02D
37/02 (20130101); F02D 41/401 (20130101); F02D
41/3029 (20130101); F02B 2275/18 (20130101); Y02T
10/44 (20130101); F02D 2041/001 (20130101); Y02T
10/18 (20130101); Y02T 10/123 (20130101); Y02T
10/12 (20130101); F02B 2075/125 (20130101); Y02T
10/125 (20130101); Y02T 10/40 (20130101) |
Current International
Class: |
F02D
37/00 (20060101); F02B 23/10 (20060101); F02D
13/02 (20060101); F02D 41/40 (20060101); F02D
41/30 (20060101); F02B 17/00 (20060101); F02D
37/02 (20060101); F02B 75/12 (20060101); F02B
75/00 (20060101); F02B 003/00 () |
Field of
Search: |
;123/294,295,298,304,305,276,279,661 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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08-177684 |
|
Jul 1996 |
|
JP |
|
11-082028 |
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Mar 1999 |
|
JP |
|
2000-303936 |
|
Oct 2000 |
|
JP |
|
Primary Examiner: Kwon; John T.
Attorney, Agent or Firm: Shinjyu Global IP Counselors,
LLP.
Claims
What is claimed is:
1. A direct fuel injection engine comprising: a combustion chamber;
a piston including a top surface having a cavity at a substantially
center portion of the top surface, the cavity being defined at
least by a peripheral wall surface and a bottom surface; a fuel
injection valve positioned at an upper portion of the combustion
chamber substantially on a center axis of the piston, the fuel
injection valve being configured and arranged to directly inject a
fuel stream inside the combustion chamber in a substantially
constant hollow circular cone shape during a compression stroke
when the direct fuel injection engine is operating in a stratified
combustion region; a spark plug configured and arranged to ignite
the fuel; and a control unit configured and arranged to control
operations of the fuel injection valve and the spark plug, the
control unit being further configured and arranged to ignite a
first air-fuel mixture formed directly after the fuel stream is
injected from the fuel injection valve and prior to a majority of
the fuel stream striking the cavity when the direct fuel injection
engine is operating in a low-load stratified combustion region, the
control unit being further configured and arranged to ignite a
second air-fuel mixture formed after a majority of the fuel stream
is guided toward an upper portion of the combustion chamber above
the cavity by the bottom surface of the cavity after the fuel
stream first hits the peripheral wall surface of the cavity when
the direct fuel injection engine is operating in a high-load
stratified combustion region.
2. The direct fuel injection engine as recited in claim 1, wherein
the control unit being further configured and arranged to ignite
the first-air fuel mixture prior to a tip of the fuel stream hits
the top surface of the piston.
3. The direct fuel injection engine as recited in claim 1, wherein
the fuel injection valve includes a plurality of nozzles to inject
a plurality of solid-core fuel streams that collectively form the
fuel stream having the substantially constant hollow circular cone
shape.
4. The direct fuel injection engine as recited in claim 1, wherein
the peripheral wall surface of the cavity is slanted radially
inwardly toward the center axis of the piston such that the cavity
forms substantially a partial cone shape.
5. The direct fuel injection engine as recited in claim 1, wherein
the bottom surface of the cavity is a substantially flat
surface.
6. The direct fuel injection engine as recited in claim 1, wherein
the control unit is further configured and arranged to set a start
timing of a fuel injection in the high-load stratified combustion
region more advanced than a start timing of a fuel injection in the
low-load stratified combustion region.
7. The direct fuel injection engine as recited in claim 1, wherein
the control unit is further configured and arranged to set a fuel
injection pressure in the high-load stratified combustion region
that is stronger than a fuel injection pressure in the low-load
stratified combustion region.
8. The direct fuel injection engine as recited in claim 1, wherein
the control unit is further configured and arranged to inject at
least one additional fuel stream during the compression stroke when
the direct fuel injection engine is operating in a relatively
high-load region within the high-load stratified combustion region,
the additional fuel stream being injected such that the additional
fuel stream first hits the bottom surface of the cavity.
9. The direct fuel injection engine as recited in claim 1, wherein
the control unit is further configured and arranged to inject the
fuel stream during an intake stroke when the direct fuel injection
engine is operating in a homogeneous combustion region in which a
load is higher than a load in the high-load stratified combustion
region.
10. The direct fuel injection engine as recited in claim 1 wherein
the control unit is configured and arranged to determine the direct
fuel injection engine is operating in the low-load stratified
combustion region when an engine load is lower than a first
prescribed engine load and an engine rotation speed is higher than
a first prescribed engine rotation speed, and the control unit is
configured and arranged to determine the direct fuel injection
engine is operating in the high-load stratified combustion region
when an engine load is higher than the first prescribed engine load
and an engine rotation speed is lower than the first prescribed
engine rotation speed.
11. The direct fuel injection engine as recited in claim 10,
wherein the control unit is configured and arranged to vary the
first prescribed engine load and the first prescribed engine
rotation speed such that as the first prescribed engine load is
increased the first prescribed engine rotation speed is
increased.
12. The direct fuel injection engine as recited in claim 10,
wherein the control unit is further configured and arranged to
determine the direct fuel injection engine is operating in the
high-load stratified combustion region when the engine load is
higher than a second prescribed engine load which is higher than
the first prescribed engine load regardless of the engine rotation
speed.
13. The direct fuel injection engine as recited in claim 10,
wherein the control unit is further configured and arranged to
determine the direct fuel injection engine is operating in the
low-load stratified combustion region when the engine load is lower
than a third prescribed engine load which is lower than the first
prescribed engine load regardless of the engine rotation speed.
14. The direct fuel injection engine as recited in claim 10,
wherein the control unit is further configured and arranged to
determine the direct fuel injection engine is operating in the
low-load stratified combustion region when the engine rotation
speed is higher than a second prescribed engine rotation speed
which is higher than the first prescribed engine rotation speed
regardless of the engine load.
15. The direct fuel injection engine as recited in claim 10,
wherein the control unit is further configured to change at least
one of fuel injection timing, fuel ignition timing, intake valve
closing timing and fuel injection pressure when the control unit
determines the direct fuel injection engine is transferring between
the low-load and high-load stratified combustion regions.
16. The direct fuel injection engine as recited in claim 15,
wherein the control unit is further configured and arranged to set
the fuel injection timing such that the fuel injection timing in
the low-load stratified combustion region is more retarded than the
fuel injection timing in the high-load stratified combustion
region.
17. The direct fuel injection engine as recited in claim 15,
wherein the control unit is further configured and arranged to set
the intake valve closing timing such that the intake valve closing
timing in the low-load stratified combustion region is more
retarded than the intake valve closing timing in the high-load
stratified combustion region.
18. The direct fuel injection engine as recited in claim 15,
wherein the control unit is further configured and arranged to set
the fuel injection pressure such that fuel injection pressure in
the low-load stratified combustion region is lower than the fuel
injection pressure in the high-load stratified combustion
region.
19. The direct fuel injection engine as recited in claims 15,
wherein the control unit is further configured and arranged to set
an interval between the fuel injection timing and the fuel ignition
timing in the low-load stratified combustion region shorter than an
interval between the fuel injection timing and the fuel ignition
timing in the high-load stratified combustion region.
20. The direct fuel injection engine as recited in claim 1, wherein
the bottom surface of the cavity is slanted such that a portion of
the bottom surface that is close to the spark plug has a depth that
is deeper than a depth of a portion of the bottom surface that is
further to the spark plug, and the peripheral wall surface of the
cavity is slanted radially inwardly toward the center axis of the
piston such that a portion of the peripheral wall surface that is
close to the spark plug is less slanted toward the center axis of
the piston than a portion of the peripheral surface that is further
to the spark plug.
21. The direct fuel injection engine as recited in claim 1, wherein
the fuel injection valve has a center axis that is slanted with
respect to the center axis of the piston, and the fuel injection
valve is configured and arranged to inject the substantially
constant hollow circular cone shape substantially symmetrical about
the center axis of the piston.
22. A direct fuel injection engine comprising: means for forming a
combustion chamber; fuel injection means for directly injecting a
fuel stream with a substantially constant hollow circular cone
shape during a compression stroke when the direct fuel injection
engine is operating in a stratified combustion region, fuel stream
guiding means for guiding the fuel stream injected from the fuel
injection means toward an upper portion of the combustion chamber;
ignition means for igniting first and second fuel mixture formed in
the combustion chamber; and control means for controlling
operations of the fuel injection means and the ignition means to
ignite the first air-fuel mixture formed directly after the fuel
stream is injected from the fuel injection means and prior to a
majority of the fuel stream striking the fuel stream guiding means
when the direct fuel injection engine is operating in a low-load
stratified combustion region, and to ignite the second air-fuel
mixture formed after a majority of the fuel stream is guided toward
the upper portion of the combustion chamber by the fuel stream
guiding means when the direct fuel injection engine is operating in
a high-load stratified combustion region.
23. A method of operating a direct fuel injection engine
comprising: injecting a fuel stream into a combustion chamber with
a substantially constant hollow circular cone shape during a
compression stroke when the direct fuel injection engine is
operating in a stratified combustion region; selectively guiding
the fuel stream toward an upper portion of the combustion chamber;
selectively igniting a first air-fuel mixture formed directly after
the fuel stream is injected into the combustion chamber and prior
to a majority of the fuel stream striking a piston when the direct
fuel injection engine is operating in a low-load stratified
combustion region; selectively igniting a second air-fuel mixture
formed after a majority of the fuel stream is guided toward the
upper portion of the combustion chamber when the direct fuel
injection engine is operating in a high-load stratified combustion
region.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a direct fuel injection
engine in which a fuel is directly injected in a combustion chamber
and ignited by a spark plug. More specifically, the present
invention relates to a direct fuel injection engine that performs
stratified combustion and homogeneous combustion by directly
injecting a fuel in the combustion chamber.
2. Background Information
One example of a conventional direct fuel injection engine is
disclosed in Japanese Laid-Open Patent Publication No. H11-82028.
The direct fuel injection engine disclosed in this reference has a
concave cavity or a piston bowl formed on the piston crown surface.
In addition, this conventional direct fuel combustion engine forms
a suitable stratified air-fuel mixture in the cylinder by arranging
a fuel injection valve substantially directly above the piston
bowl. This arrangement allows the fuel stream to collide against a
peripheral side wall of the piston bowl and form a fuel stream
circulation flow towards the center portion of the piston bowl to
reduce fuel consumption.
In view of the above, it will be apparent to those skilled in the
art from this disclosure that there exists a need for an improved
direct fuel injection engine. This invention addresses this need in
the art as well as other needs, which will become apparent to those
skilled in the art from this disclosure.
SUMMARY OF THE INVENTION
In the conventional direct fuel injection engine described above, a
volume of the stratified air-fuel mixture that is formed after
colliding with the piston is determined substantially by the shape
of the piston cavity or the capacity of the cavity. In other words,
in the above mentioned conventional direct fuel injection engine,
the volume of the stratified air-fuel mixture formed via the cavity
is always constant regardless of the engine load since the capacity
of the cavity is constant. Therefore, a range of engine load
conditions that allows excellent stratified combustion operation is
limited with the conventional direct fuel injection engine. More
specifically, if the cavity size and other control parameter is
determined to obtain stable combustion and to realize good fuel
efficiency and small exhaust gas emissions during one stratified
operating region with a certain engine load and a certain engine
rotation speed, then good fuel efficiency may not be realized or
the so-called recoil will occur in another stratified operating
region with a different engine rotation speed and a different
engine load.
For example, when the engine rotation speed is fast, the advance of
the crank angle becomes faster compared to when the engine rotation
speed is slow, and thus, the time allowed to form an air-fuel
mixture becomes shorter. Therefore, if an identical fuel injection
timing and an identical fuel injection duration (this is basically
proportional to the engine load) are used for both when the
rotation speed is fast and slow, then the ignition timing occurs
before the combustible air-fuel mixture reaches the vicinity of the
spark plug when the engine rotation speed is fast. In order to
avoid this problem, it is possible to set the fuel injection timing
to be more advanced as the engine rotation speed becomes. However,
in such case, the fuel stream injected during an earlier part of
the injection might not be received in the cavity of the piston. In
particular, considering executing a homogeneous combustion in the
full load region, in which the fuel is injected during an intake
stroke, the fuel stream must be injected with at least equal to or
more than a certain injection opening angle. Thus, the fuel
injection timing in the stratified combustion state cannot be
arranged to be too early.
Moreover, when the cavity is made with a larger opening in order to
make it easier to accept the fuel stream, the depth of the cavity
is restricted to be less than a certain depth in view of the
compression ratio. Accordingly, it becomes difficult to receive the
fuel stream due to an insufficient depth.
Thus, since an air-fuel mixture is always formed via the cavity and
ignited during stratified combustion in a conventional direct fuel
injection engine, it is difficult to obtain stable combustion and
good fuel efficiency as well as small exhaust emission under
various conditions in which the engine rotation speed fluctuates
between fast and slow. Moreover, the engine load also changes from
low-load to high-load or vice versa during the stratified
combustion. When the engine load is low during the stratified
combustion, the stratified air-fuel mixture in the vicinity of the
spark plug tends to be lean in the above mentioned conventional
direct fuel injection engine because the air-fuel mixture is formed
after the fuel stream collides against the cavity. Thus, the
combustion stability is worsened thereby causing the fuel
efficiency to deteriorate. On the other hand, when the engine load
is high during the stratified combustion, the air-fuel mixture in
the vicinity of the spark plug tends to become excessively dense
with the above mentioned conventional direct fuel injection engine.
Thus, smoke and HC is increased.
Accordingly, one object of the present invention is to expand a
range of engine operation conditions that allows excellent
stratified combustion operation at a low cost by executing two
different stratified combustion operations depending on the engine
load and/or the engine rotation speed.
In order to achieve the above and other objects, a direct fuel
injection engine of the present invention basically comprises a
combustion chamber, a piston, a fuel injection valve, a spark plug
and a control unit. The piston includes a top surface having a
cavity at a substantially center portion of the top surface. The
cavity is defined at least by a peripheral wall surface and a
bottom surface. The fuel injection valve is positioned at an upper
portion of the combustion chamber substantially on a center axis of
the piston. The fuel injection valve is configured and arranged to
directly inject a fuel stream inside the combustion chamber in a
substantially constant hollow circular cone shape during a
compression stroke when the direct fuel injection engine is
operating in a stratified combustion region. The spark plug is
configured and arranged to ignite the fuel. The control unit is
configured and arranged to control operations of the fuel injection
valve and the spark plug. The control unit is further configured
and arranged to ignite a first air-fuel mixture formed directly
after the fuel stream is injected from the fuel injection valve and
prior to a majority of the fuel stream striking the cavity when the
direct fuel injection engine is operating in a low-load stratified
combustion region. The control unit is further configured and
arranged to ignite a second air-fuel mixture formed after a
majority of the fuel stream is guided toward an upper portion of
the combustion chamber above the cavity by the bottom surface of
the cavity after the fuel stream first hits the peripheral wall
surface of the cavity when the direct fuel injection engine is
operating in a high-load stratified combustion region.
These and other objects, features, aspects and advantages of the
present invention will become apparent to those skilled in the art
from the following detailed description, which, taken in
conjunction with the annexed drawings, discloses preferred
embodiments of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the attached drawings which form a part of this
original disclosure:
FIG. 1 is a partial cross sectional view of an injection portion of
a combustion chamber of a direct fuel injection engine in
accordance with a first embodiment of the present invention;
FIG. 2 is a diagrammatic chart illustrating the relationship
between engine operation regions and an engine load and an engine
rotation speed in accordance with the first embodiment of the
present invention;
FIG. 3(a) is a diagrammatic cross sectional view of the combustion
chamber shown in FIG. 1 illustrating distribution of the air-fuel
mixture in the combustion chamber in a low-load stratified
combustion region in accordance with the first embodiment of the
present invention;
FIG. 3(b) is an enlarged, partial diagrammatic side view of a spark
plug and a fuel stream illustrating distribution of the fuel stream
in a time-series manner in the low-load stratified combustion
region in accordance with the first embodiment of the present
invention;
FIG. 4 is a diagrammatic cross sectional view of the combustion
chamber shown in FIG. 1 illustrating distribution of the air-fuel
mixture in the combustion chamber in a high-load stratified
combustion region in accordance with the first embodiment of the
present invention;
FIG. 5 is a diagrammatic chart illustrating the relationship
between engine operation regions and an engine load and an engine
rotation speed in accordance with a second embodiment of the
present invention;
FIG. 6 is a diagrammatic chart illustrating the relationship
between engine operation regions and an engine load and an engine
rotation speed in accordance with the second embodiment of the
present invention;
FIG. 7(a) is a diagrammatic cross sectional view of the combustion
chamber shown in FIG. 1 illustrating distribution of the air-fuel
mixture in the combustion chamber in a high-load stratified
combustion region when an engine load is relatively high in
accordance with the second embodiment of the present invention;
FIG. 7(b) is a diagrammatic cross sectional view of the combustion
chamber shown in FIG. 1 illustrating distribution of the air-fuel
mixture in the combustion chamber in a high-load stratified
combustion region when an engine load is relatively low in
accordance with the second embodiment of the present invention;
FIG. 8 is a diagrammatic chart illustrating a change in control
parameters including fuel pressure, intake valve closing timing,
fuel injection timing, and fuel ignition timing between the
high-load and low-load stratified combustion regions with respect
to the engine load in accordance with the second embodiment of the
present invention;
FIG. 9 is a diagrammatic chart illustrating a change in control
parameters including fuel pressure, intake valve closing timing,
fuel injection timing, and fuel ignition timing between the
high-load and low-load stratified combustion regions with respect
to the engine rotation speed in accordance with the second
embodiment of the present invention;
FIG. 10 is a diagrammatic cross sectional view of a combustion
chamber illustrating distribution of the air-fuel mixture in the
combustion chamber in accordance with a third embodiment of the
present invention;
FIG. 11 is a diagrammatic chart illustrating the relationship
between engine operation regions and an engine load and an engine
rotation speed in accordance with the third embodiment of the
present invention;
FIG. 12 is a partial cross sectional view of an injection portion
of a combustion chamber of a direct fuel injection engine in
accordance with a fourth embodiment of the present invention;
and
FIG. 13 is a partial cross sectional view of an injection portion
of a combustion chamber of a direct fuel injection engine in
accordance with a fifth embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Selected embodiments of the present invention will now be explained
with reference to the drawings. It will be apparent to those
skilled in the art from this disclosure that the following
descriptions of the embodiments of the present invention are
provided for illustration only and not for the purpose of limiting
the invention as defined by the appended claims and their
equivalents.
Referring initially to FIGS. 1-4, a direct fuel injection engine is
illustrated in accordance with a first embodiment of the present
invention. FIG. 1 is a partial cross sectional view of an injection
portion of a combustion chamber 1 of a direct fuel injection engine
of the first embodiment. The combustion chamber 1 is basically
formed by a cylinder head 2a, a cylinder block 2b and a piston 3. A
gasket 14 is placed between the cylinder head 2a and the cylinder
block 3. A substantially cylindrical cavity 4 is provided at the
center of a crown surface or a top surface of the piston 3. The
cavity 4 is formed with a peripheral wall surface or an inner
peripheral surface 4a and a flat bottom surface 4c that are
smoothly joined by a curved surface 4b. As seen in FIG. 1, the
inner peripheral surface 4a is preferably inclined or slanted
towards a center axis of the piston 3 to form a reentrant shape of
the cavity 4. The cavity flat bottom surface 4c is preferably a
smooth surface without any unevenness and disposed substantially
perpendicular to the center axis of the piston 3. Therefore, the
cavity 4 forms a substantially cone shape with a portion including
an apex of the cone being cut off. An intake port 5 is arranged to
send air required for combustion to the combustion chamber 1
through an intake valve 7 that is operatively controlled by an
intake valve cam 9. The intake valve 7 is preferably coupled to a
variable valve timing mechanism that allows at least the intake
valve closing timing to be varied (i.e., delayed and advanced). For
example, a variable valve timing mechanism that changes the
relative phase between the camshaft and the crankshaft can be
coupled to the intake valve 7. Such variable valve timing mechanism
is well known in the art, and thus, not discussed in detail herein.
An exhaust port 6 discharges exhaust gases combusted in the
combustion chamber 1 through an exhaust valve 8 that is operatively
controlled by an exhaust valve cam 10.
A spark plug 12 is positioned substantially adjacent to the fuel
injection valve 11 so that a spark gap 12a of the spark plug 12 is
positioned in the vicinity of the center of the combustion chamber
1. The spark plug 12 is configured and arranged to ignite the fuel
stream injected by the fuel injection valve 11 to cause
combustion.
A fuel injection valve 11 is provided on the cylinder head 2a and
preferably positioned substantially on the center axis of the
piston 3, which is substantially coincident with a center axis of a
cylinder, at the upper portion of the combustion chamber 1. The
fuel injection valve 11 is preferably configured and arranged to
have a plurality of through-holes or nozzles with identical shapes
through which the fuel is injected into the combustion chamber 1.
More specifically, a plurality of solid-core fuel streams is
injected from these nozzles towards the piston 3. Thus, the
plurality of solid-core fuel streams injected from the fuel
injection valve 11 collectively forms a fuel stream with a
substantially constant hollow cone shape. The "substantially
constant hollow cone shape" as used herein basically refers to that
an injection opening angle or an apex angle (umbrella angle) of the
cone is substantially constant. In other words, the fuel injection
valve 11 does not have a special variable mechanism for changing
the injection opening angle. Rather, the present invention utilizes
the fuel injection valve 11 whose injection opening angle is not
greatly affected by the factors such as an amount of fuel injected
or fuel injection timing (i.e., pressure inside the cylinder during
fuel injection). Therefore, the fuel stream can be reliably
injected toward a desired direction. More specifically, in the
present invention, the fuel injection valve 11 is configured and
arranged to inject the fuel stream toward the discharge gap 12a of
the spark plug 12 or in the vicinity of the electrodes of the spark
plug 12 when the engine is operating in the low-load stratified
combustion state or region, and toward the cavity inner peripheral
surface 4a when the engine is operating in the high-load stratified
state or region.
An engine control unit 13 is operatively coupled to the spark plug
12 and the fuel injection valve 11 and configured and arranged to
control various operations of the direct fuel injection engine,
such as the fuel injection timing and duration of the fuel
injection valve 11 and the fuel injection timing of the spark plug
12, based on the engine operational conditions. More specifically,
the control unit 13 preferably includes a microcomputer with a
control program that controls the direct fuel injection engine as
discussed below. The control unit 13 can also include other
conventional components such as an input interface circuit, an
output interface circuit, and storage devices such as a ROM (Read
Only Memory) device and a RAM (Random Access Memory) device. The
microcomputer of the control unit 13 is programmed to control the
direct fuel control engine. The memory circuit stores processing
results and control programs that are run by the processor circuit.
The control unit 13 is operatively coupled to the various
components of the direct fuel injection engine including the fuel
injection valve 11 and the spark plug 12 in a conventional manner.
The internal RAM of the control unit 13 stores statuses of
operational flags and various control data. The control unit 13 is
capable of selectively controlling any of the components of the
control system in accordance with the control program. It will be
apparent to those skilled in the art from this disclosure that the
precise structure and algorithms for control unit 13 can be any
combination of hardware and software that will carry out the
functions of the present invention. In other words, "means plus
function" clauses as utilized in the specification and claims
should include any structure or hardware and/or algorithm or
software that can be utilized to carry out the function of the
"means plus function" clause.
The direct fuel injection engine of the present invention is
configured and arranged to perform combustion of the air-fuel
mixture in a homogeneous combustion operating region or a
stratified combustion operating region depending on an operating
condition of the direct fuel injection engine. In the homogeneous
combustion operating region, the fuel is injected during an intake
stroke (preferably in the first half of the intake stroke) to form
a homogeneous fuel air mixture throughout the combustion chamber 1
to perform combustion in a stoichiometric air-fuel ratio operation.
In the stratified combustion operating region, a fuel is injected
during a compression stroke (preferably in the second half of the
compression stroke) to form a stratified fuel-air mixture inside
and/or above the cavity 4 to achieve a lean operation to improve
fuel economy. Moreover, in the first embodiment of the present
invention, two different operations are performed in the stratified
combustion operating region depending on the engine load. In a
low-load stratified combustion region, the fuel injected from the
fuel injection valve 11 is ignited directly after the fuel is
injected before a majority of the fuel stream collides against the
cavity 4. In a high-load stratified combustion region, the fuel
stream is ignited after the fuel stream collides against the inner
peripheral surface 4a of the cavity 4 and rises upwardly in the
center portion of the cavity 4 as guided by the curved surface 4b
and the cavity bottom surface 4c. Moreover, by utilizing the fuel
injection valve 11 that injects a fuel stream with a substantially
constant hollow cone shape, a comparatively small air-fuel mixture
mass in a low-load stratified combustion region and a comparatively
large air-fuel mixture mass in a high-load stratified combustion
region are obtained at a low cost. Accordingly, with the
arrangement of the direct fuel injection engine of the present
invention, the stratified combustion operating region can be
expanded at a low cost.
FIG. 2 is a diagrammatic chart illustrating the relationship
between the homogeneous combustion operating region, the high-load
stratified combustion region and the low-load stratified combustion
region with respect to an engine load and an engine rotation speed
in accordance with the first embodiment. Among all of the operating
regions, the stratified combustion operating region (including the
high-load and low-load stratified combustion regions) is set to a
comparatively low load and slow rotational speed region. In the
stratified combustion operating region, a stratified combustion is
performed in which an air-fuel mixture is formed within a portion
above the cavity 4 and/or inside the cavity 4. As seen in FIG. 2,
the stratified combustion operating region is divided into the
high-load stratified combustion region where the engine load is
relatively high and the low-load stratified combustion region where
the engine load is relatively low.
When the direct fuel injection engine is operating in the low-load
stratified combustion region, the control unit 13 is configured and
arranged to operate the spark plug 12 to ignite an air-fuel mixture
that is formed directly after the fuel stream is injected from the
fuel injection valve 11 and before the majority of the fuel stream
collides against the cavity 4. When the direct fuel injection
engine is operating in the high-load stratified combustion region,
the control unit 13 is configured and arranged to operate the spark
plug 12 to ignite an air-fuel mixture formed after a majority of
the fuel stream is guided toward an upper portion of the combustion
chamber 1 above the cavity 4 by the bottom surface 4c of the cavity
4 after the fuel stream first collides against the inner peripheral
wall surface 4a of the cavity 4. In other words, an interval
between when the fuel is injected and when the fuel is ignited is
set relatively shorter in the low-load stratified combustion region
and relatively longer in the high-load stratified combustion
region.
As seen in FIG. 2, the homogeneous combustion operating region is
set such that the engine load is higher and the engine rotation
speed is faster in the homogeneous combustion operating region than
in the stratified combustion operating region. In the homogeneous
combustion operating region, a fuel is injected from the fuel
injection valve 11 during an intake stroke. Air is introduced from
the intake port 5 to the combustion chamber 1 to form a homogeneous
air-fuel mixture throughout the entire combustion chamber 1 so that
homogeneous combustion is performed.
Referring now to FIGS. 3(a) and 3(b), distribution of the air-fuel
mixture (first air-fuel mixture) in the combustion chamber 1 in the
low-load stratified combustion region is described. In the low-load
stratified combustion region, the fuel injection pressure is set to
a comparatively low pressure. Thus, the penetration force of the
fuel stream is reduced and a size of the air-fuel mixture mass is
also reduced. Moreover, in the low-load stratified combustion
region, the fuel injection timing is set to inject the fuel stream
during the second half of the compression stroke close to the
compression top dead center. Thus, a relatively small air-fuel
mixture is ignited when the crank angle is close to the compression
top dead center. Consequently, the fuel consumption efficiency is
improved.
Since the amount of fuel injected in the low-load stratified
combustion region is set to a small amount, the fuel injection is
preferably completed before the tip of the hollow cone shape fuel
stream reaches the piston 3. Thus, the air-fuel mixture is ignited
by the spark plug 12 during the fuel injection or directly after
the fuel injection is completed before the tip of the fuel stream
reaches the cavity 4. More specifically, the fuel ignition timing
is preferably set such that the air-fuel mixture is ignited when
the fuel stream is still floating in the air before the tip of the
hollow cone shape fuel stream reaches the piston 3. Accordingly, as
seen in FIG. 3(a), a size of the air-fuel mixture mass at the time
of ignition is relatively small in the low-load stratified
combustion region.
The injection opening angle .theta. of the fuel injection valve 11
is preferably set to a relatively wide angle, for example, from
approximately 60.degree. to approximately 80.degree.. Thus, the
fuel stream injected from the fuel injection valve 11 preferably
passes through the discharge gap 12a of the spark plug 12 or a
through an area that is close to the electrodes of the spark plug
12. This arrangement of the relatively wide injection opening angle
.theta. of the fuel injection valve 11 is advantageous because the
fuel stream can be ignited by the spark plug 12 directly after the
fuel injection ends as explained above. If the injection opening
angle .theta. of the fuel injection valve 11 is relatively narrow,
the discharge gap 12a of the spark plug 12 must protrude deeper
into the combustion chamber 1 in order to ignite the fuel stream
directly after injected, which makes it difficult to ensure the
durability of the spark plug 12.
As seen in FIG. 3(b), the injected fuel stream is mixed with the
surrounding air from the tip or periphery of the fuel stream. By
setting the fuel injection timing close to the compression top dead
center, the temperature inside the cylinder during fuel injection
is high. Thus, the fuel is quickly vaporized and mixed with the air
after injected. Accordingly, in the low-load stratified combustion
region, the air-fuel mixture formed in the periphery of the fuel
stream is ignited during the fuel injection or directly after the
fuel injection is completed as the fuel stream is quickly vaporized
and mixed with the air. Consequently, stable combustion is
performed in the low-load stratified combustion region even when
the engine rotation speed is fast and sufficient time cannot be
provided to form an air-fuel mixture after the fuel stream collides
against the cavity 4. Moreover, when the engine load is low and the
amount of fuel injected is small, the density of the air-fuel
mixture formed by colliding the fuel stream against the piston 3
and defusing the fuel stream inside the cavity 4 generally tends to
be excessively thin. Thus, since the fuel is directed ignited after
injected with the direct fuel injection engine of the present
invention, stable combustion is performed in the low-load
stratified combustion region even when the engine load is low and
the amount of fuel injected is relatively small. In addition,
because the fuel is ignited before the tip of the fuel stream makes
contact with the crown surface of the piston 3, the fuel is
combusted without adhering to the piston 3. Thus, the amount of
unburned HC produced is reduced. Moreover, the air-fuel mixture is
combusted in a state in which the thermal insulation layer (air
layer) is positioned between the air-fuel mixture and the piston 3,
and thus, cooling loss is reduced.
Next, referring to FIG. 4, distribution of the air-fuel mixture
(second air-fuel mixture) in the combustion chamber 1 in the
high-load stratified combustion region is described. In the
high-load stratified combustion region, the fuel injection pressure
is set to a comparatively high pressure. Thus, the penetration
force of the fuel stream is increased thereby making it possible to
produce a strong circulation flow of the fuel stream. Moreover, it
is further preferable to set the fuel injection pressure higher as
the engine load grows larger while the engine is operating in the
high-load stratified combustion region. Furthermore, the fuel
injection timing in the high-load stratified combustion region is
set to the second half of the compression stroke that is more
advanced than the fuel injection timing in the low-load stratified
combustion region. Consequently, a sufficient time is ensured to
mix the fuel and air by utilizing a circulation flow.
Since the fuel injection valve 11 is configured to inject a fuel
stream to form a substantially constant hollow cone shape, the
shape of the fuel stream in the high-load stratified combustion
region is substantially identical to the shape of the fuel stream
in the low-load stratified combustion region (i.e., the injection
opening angle .theta. is from approximately 60.degree. to
approximately 80.degree.). Thus, the fuel stream injected from the
fuel injection valve 11 reaches the piston 3 and collides against
the inner peripheral surface 4a of the cavity 4 as seen in diagram
(A) of FIG. 4. Since the cavity inner peripheral surface 4a is
preferably inclined inwardly such that the cavity 4 forms an
substantially cone shape, the collision angle between the cavity
inner peripheral surface 4a and the fuel stream is relatively small
(e.g., an acute angle). Thus, majority of the fuel stream is guided
downwardly toward the cavity 4 and remained within or above the
cavity 4. Of course, it will be apparent to those skilled in the
art from this disclosure to modify the cavity inner peripheral
surface 4a to extend substantially parallel to the center axis of
the piston 3 or slightly inclined radially outwardly in order to
make the fabrication of the cavity 4 easier, although the amount of
fuel overflowing from the cavity 4 will increase slightly in such
cases.
After the collision, the fuel stream is guided downwardly in the
cavity 4 along the cavity inner peripheral surface 4a. The travel
direction of the fuel stream is then curved inwardly by the cavity
curved surface 4b. Then, the fuel travels towards radial inner
direction of the cavity 4 along the cavity bottom surface 4c. Since
the cavity bottom surface 4c is preferably formed without any
unevenness on its surface, the fuel stream traveling from the
radial peripheral direction to the center of the cavity bottom
surface 4c collide against each other in the vicinity of the center
of the cavity bottom surface 4c. Thus, a flow that rises upwardly
is effectively formed in the cavity 4 as seen in diagram (B) of
FIG. 4. This flow of the fuel stream results in the fuel stream
encompassing the circumference air as the fuel streams travels.
Thus, a circulation flow of air-fuel mixture within the space
between the cylinder head 2a and the piston 3 is formed as seen in
diagram (C) of FIG. 4. This circulation flow promotes mixing of
fuel and air and creates a substantially homogeneous air-fuel
mixture mass within the cavity 4 and thereabove. In other words,
when the fuel streams collide in the vicinity of the center of the
bottom surface 4c, a moderate disturbance occurs resulting in
favorable mixing of the fuel and air and form a substantially
homogeneous air-fuel mixture within the cavity 4 and thereabove.
Then, the spark plug 12 is configured and arranged to ignite this
substantially homogeneous air-fuel mixture mass. Accordingly, in
the high-load stratified combustion region, the size of the
air-fuel mixture mass at the time of fuel ignition is relatively
large.
The arrangement of the relatively wide injection opening angle
.theta. of the fuel injection valve 11 is advantageous in the
high-load stratified combustion region as well as in the low-load
stratified combustion region. If a fuel injection valve 11 with a
narrow injection opening angle is used, the fuel stream collides
with the cavity bottom surface 4c resulting in a circulation flow
that rotates in a direction opposite to the circulation flow
described above. In such case, an air-fuel mixture mass created
within the cavity 4 and thereabove tends to have a less dense
air-fuel ratio at the center of this air-fuel mixture mass that is
close to the spark plug 12. On the other hand, since the injection
opening angle .theta. of the fuel injection valve 11 is relatively
wide in the present invention, the air-fuel mixture mass can be
obtained in which the air-fuel ratio is substantially uniform
throughout the mass or the air-fuel ratio is slightly more dense at
the center of the mass and becomes less dense as moving towards the
periphery of the mass. Accordingly, with the direct fuel injection
engine of the present invention, an effective ignition and stable
combustion are obtained. Consequently, EGR can be introduced in
large quantities making it possible to operate the engine with a
small amount of NOx occurring. In the high-load stratified
combustion region, the fuel may adhere to the cavity 4 of the
piston 3 when the fuel stream collides against the cavity 4.
However, the circulation flow of air-fuel mixture within the cavity
4 promotes the vaporization of the adhering fuel. Thus, any sudden
increase in the amount of unburned HC produced is prevented in the
high-load stratified combustion region.
Next, the fuel injection and the distribution of the air-fuel
mixture during the homogeneous combustion region will be described.
During the homogeneous combustion region, the fuel is injected from
the fuel injection valve 11 in the second half of the intake
stroke. As in the stratified combustion region, the fuel injection
valve 11 is configured and arranged to inject a fuel stream with
the relatively wide injection opening angle .theta.. Thus, the fuel
stream is diffused throughout the combustion chamber 1 including
areas outside of the cavity 4 and thoroughly mixed with the air to
create an air-fuel mixture having a substantially stoichiometric
air-fuel ratio in the combustion chamber 1. Accordingly, the
combustion with good fuel efficiency and a small amount of exhaust
gas emissions is achieved.
Accordingly, by changing the operations in the stratified
combustion region depending on the engine load and by utilizing the
fuel injection valve 11 that injects a fuel stream with a
substantially constant hollow cone shape, the direct fuel injection
engine of the present invention enables to obtain an excellent
combustion in a wide range of engine operating conditions.
SECOND EMBODIMENT
Referring now to FIGS. 5-8, a direct fuel injection engine in
accordance with a second embodiment will now be explained. In view
of the similarity between the first and second embodiments, the
parts of the second embodiment that are identical to the parts of
the first embodiment will be given the same reference numerals as
the parts of the first embodiment. Moreover, the descriptions of
the parts of the second embodiment that are identical to the parts
of the first embodiment may be omitted for the sake of brevity.
Basically, the direct fuel injection engine of the second
embodiment is identical to the direct fuel injection engine of the
first embodiment, except that operation of the direct fuel
injection engine is in either the low-load or high-load stratified
combustion regions is determined based on the engine rotation speed
as well as the engine load. Moreover, in the second embodiment of
the present invention, the operations of the intake valve opening
timing and the fuel injection timing as well as the operations of
the fuel injection pressure and the fuel injection timing are
changed depending on whether the direct fuel injection engine is
operating in the low-load or high-load stratified combustion
regions.
When the engine rotation speed increases in the high-load
stratified combustion region, there is a concern that there is no
sufficient time to form a homogeneous air-fuel mixture. Moreover,
there is also a danger of the flow inside the cylinder becoming so
strong that the air-fuel mixture is excessively diffused and the
air-fuel ratio in the vicinity of the spark plug 12 becomes
excessively thin. On the other hand, if an excessive amount of fuel
is injected during the low-load stratified combustion and the fuel
is ignited while the fuel is being vaporized and mixed with the
air, then an excessively dense air-fuel mixture may exist during
flame propagation. In such case, the fuel combustion efficiency is
reduced and exhaust gas emissions is increased.
Thus, in the second embodiment of the present invention, the
operation in the low-load stratified combustion is executed when
the engine rotation speed is relatively fast so that an air-fuel
mixture with an air-fuel ratio suitable for ignition is always
formed near the spark plug 12 regardless of the engine rotation
speed. Generally, the gas flow inside the cylinder becomes larger
as the engine rotation speed becomes faster. When the engine
rotation speed is fast, the fuel stream is diffused and mixed with
the air relatively fast due to the relatively large gas flow inside
the cylinder. Thus, mixing of fuel is promoted in the flame
propagation process in the low-load stratified combustion region
after the fuel stream is injected due to the flow inside the
cylinder when the engine rotation speed is relatively fast. Thus,
the formation of an excessively dense air-fuel mixture is prevented
in the low-load stratified combustion region. When, however, the
engine rotation speed is relatively slow and the engine load is in
relatively high, it is preferable to provide sufficient time for
the fuel stream to be vaporized and mixed with the air.
Accordingly, in the second embodiment of the present invention, the
control unit 13 is configured to consider an engine rotation speed
as well as an engine load in determining the operational regions of
the direct fuel injection engine as seen in FIG. 5. More
specifically, the control unit 13 is configured and arranged to
determine the direct fuel injection engine is operating in the
high-load stratified combustion region when the engine rotation
speed is slower than a first prescribed engine rotation speed and
the engine load is higher than a first prescribed engine load. The
control unit 13 is configured to determine the direct fuel
injection engine is operating in the low-load stratified combustion
region when the engine rotation speed is faster than the first
prescribed engine rotation speed and the engine load is lower than
the first prescribed engine load. As seen in FIG. 5, the first
prescribed engine load and the first prescribed engine rotation
speed are preferably set such that the higher the first prescribed
engine load becomes, the faster the first prescribed engine
rotation speed becomes. In the second embodiment of the present
invention, the homogeneous combustion is also performed in which
the fuel is injected during a intake stroke in the homogeneous
combustion region in which the engine load is equal to or more than
a predetermined engine load and the engine rotation speed is equal
to or more than a predetermined engine rotation speed as shown in
FIG. 5.
Also, as seen in FIG. 6, under certain engine operation conditions,
the control unit 13 is configured to determine the direct fuel
injection engine is operating in the high-load or low-load
stratified combustion engine regardless of the engine rotation
speed. Specifically, when the engine load increases more than a
certain load (a second prescribed engine load), sufficient mixing
of the fuel and air becomes difficult even though the flow is
intensified by increasing the engine rotation speed if the fuel is
ignited during fuel injection or directly after completion of the
fuel injection but before the fuel stream collides against the
piston 3 (i.e., the operation in the low-load stratified combustion
region). In other words, if the control unit 13 determines the
direct fuel injection engine is operating in the low-load
stratified combustion engine when the engine load is equal to or
higher than the second prescribed engine load, a risk increases
that an excessively dense air-fuel mixture will be partially
formed. Therefore, as shown in FIG. 6, the control unit 13 is
configured to determine the direct fuel injection engine is
operating in the high-load stratified operating region when the
engine load is equal to or more than the second prescribed engine
load regardless of the engine rotation speed. Thus, when the engine
load is equal to or more than the second prescribed engine load,
the fuel stream is ignited after colliding against the cavity 4 and
forming an air-fuel mixture inside and above the cavity 4.
Moreover, when the engine load is equal to or less than a certain
load (a third prescribed engine load), the amount of the fuel
injected from the fuel injection valve 11 becomes small. Therefore,
there is a risk of the density of the air-fuel mixture becoming
excessively thin if the fuel is ignited after colliding against the
cavity 4 and forming the air-fuel mixture as the operation in the
high-load stratified combustion region. Accordingly, the control
unit 13 is configured to determine the direct fuel injection engine
is operating in the low-load stratified combustion region when the
engine load is equal to or lower than the third prescribed engine
load regardless of the engine rotation speed, as seen in FIG. 6.
Thus, when the engine load is equal to or lower than the third
prescribed engine load, the fuel stream is ignited during fuel
injection or directly after completion of the fuel injection but
before the fuel stream collides against the piston 3.
Furthermore, the control unit 13 is configured to determine the
direct fuel injection engine is operating in the low-load
stratified combustion region regardless of the engine load under
certain engine operation conditions. When the engine rotation speed
becomes faster than a certain rotation speed (second engine
rotation speed), there is a risk that sufficient time is not
provided to form an air-fuel mixture by hitting the fuel stream
against the cavity 4. Moreover, in such case, if the fuel injection
timing is set to reliability receive the fuel stream in the cavity
4, there is a risk that the air-fuel mixture has not reached near
the spark plug 12 when the fuel ignition timing occurs. Therefore,
the control unit 13 is configured to determine the direct fuel
injection engine is operating in the low-load stratified combustion
region when the engine rotation speed is faster than the second
prescribed engine rotation speed regardless of the engine load, as
shown in FIG. 6. Thus, when the engine rotation speed is faster
than the second prescribed engine rotation speed, the fuel stream
is ignited during fuel injection or directly after the fuel
injection completes before the fuel stream collides against the
piston 3.
Moreover, the control unit 13 is configured and arranged to change
operation parameters including the intake valve closing timing, the
fuel injection pressure, the fuel injection timing and the fuel
ignition timing depending on whether the direct fuel injection
engine is operating in the high-load or low-load stratified
combustion regions. As seen in FIG. 7(a), when the direct fuel
injection engine is operating in the high-load stratified
combustion region and when the engine load is relatively high
within the high-load stratified region, a relatively dense
homogeneous air-fuel mixture is formed in the space above the
cavity 4. However, as the engine load becomes smaller in the
high-load stratified combustion region, the density of the air-fuel
mixture formed in the space above the cavity 4 becomes thinner as
shown in FIG. 7(b). Thus, the second embodiment of the present
invention is configured to control the fuel injection timing and
the fuel ignition timing of the direct fuel injection engine to
keep the density of the air-fuel mixture formed inside the
combustion chamber within an air-fuel ratio which is combustible
and which does not worsen the exhaust gas emissions. In other
words, under the conditions where the engine load is so high that
the density of the homogeneous air-fuel mixture formed in the space
above ihe cavity 4 becomes very dense which results in worsening
the exhaust emission, the fuel injection timing and fuel ignition
timing is controlled so that a part of the air-fuel mixture is
drawn to the outside of the cavity 4. Under the conditions where
the density of the homogeneous air-fuel mixture formed in the space
above the cavity 4 becomes so lean that there is a danger that an
accidental combustion may occur, the fuel injection timing and
ignition timing is controlled such that an air-fuel mixture with a
mild air-fuel ratio distribution is ignited before the air-fuel
mixture spreads throughout the entire space above the cavity 4.
More specifically, FIG. 8 illustrates variations in each parameter
with respect to the engine load assuming the engine rotation speed
is constant. FIG. 9 illustrates variations in each parameter with
respect to the engine rotation speed assuming the engine load is
constant. As shown in FIGS. 8 and 9, the fuel injection timing in
the low-load stratified combustion region is set to be more
retarded with respect to the fuel injection timing in the high-load
stratified combustion region. Moreover, in both the low-load and
high-load stratified combustion regions, the fuel injection timing
is controlled such that the fuel injection timings is more advanced
as the engine load becomes higher or the engine rotation speed
becomes faster.
Moreover, the fuel ignition timing in the low-load stratified
combustion region is also set more retarded with respect to the
fuel injection timing in the high-load stratified region. However,
an amount of change in fuel ignition timing between the low-load
and high-load stratified combustion regions is kept smaller than an
amount of change in fuel injection timing between the low-load and
high-load stratified combustion regions. In other words, as seen in
FIGS. 8 and 9, intervals T1 and T2 between the fuel injection
timing and the fuel ignition timing in low-load and high-load
stratified combustion regions, respectively, are preferably set
such that the interval T1 of the low-load stratified combustion
region is shorter than the interval T2 of the high-load stratified
combustion region. This arrangement of the control parameters
provides reliable ignition of the fuel stream that is floating in
the combustion chamber 1 before the fuel stream collides against
the piston 3 in the low-load stratified combustion region.
In FIGS. 8 and 9, the fuel ignition timing is simplified to be
substantially constant within the high-load or low-load stratified
combustion region. However, it will be obvious to one of ordinary
skill in the art from this disclosure that the fuel ignition timing
can be varied in each stratified combustion region as the fuel
ignition timing is preset to realize optimum fuel efficiency and
exhaust emission as explained above.
Moreover, the intake valve closing timing in the low-load
stratified combustion region is more retarded with respect to the
intake valve closing timing in the high-load stratified combustion
region. Accordingly, the pressure inside the cylinder during the
fuel injection timing is lowered in the low-load stratified
combustion region. Thus, the actual fuel stream angle (injection
opening angle) of the fuel stream in the low-load stratified
combustion region expands slightly due to the pressure differential
between the high-load and low-load combustion regions. Thus, the
fuel stream is further reliably directed in the vicinity of the
spark plug 12 to achieve stable combustion.
Furthermore, the fuel injection pressure (fuel pressure) in the
low-load stratified combustion region is set lower than the fuel
injection pressure in the high-load stratified combustion region.
Thus, the travel speed of the fuel stream in the low-load
stratified combustion region is reduced such that the air-fuel
mixture formed around the fuel stream main axis remains in the
vicinity of the spark plug 12. Thus, even more reliable and stable
combustion can be achieved in the low-load stratified combustion
region.
According to the second embodiment of the present invention, the
operation of the direct fuel injection engine is switched between
the low-load stratified combustion region and the high-load
stratified combustion region depending on the engine load and
engine rotation speed to provide sufficient time required to form
the air-fuel mixture, and thus, stable stratified combustion is
obtained regardless of the engine rotation speed.
Since the air-fuel mixture is formed and ignited after the fuel
stream hits against the cavity 4 and guided upwardly above the
cavity 4 in the high-load stratified combustion region, the fuel is
sufficiently vaporized and mixed with the air to form a relatively
homogeneous air-fuel mixture in the space above of the cavity 4.
Thus, smoke or CO discharge from an excessively dense air-fuel
mixture can be reduced and the engine operation is prevented from
being affected by the fluctuations in the cycle of the air-fuel
mixture distribution.
However, when the engine rotation speed increases in the high-load
stratified combustion region, there is a concern that the time may
be insufficient to form a homogeneous air-fuel mixture. Moreover,
there is also a danger of the flow inside the cylinder becoming too
strong that the air-fuel mixture is excessively diffused and the
area near the spark plug 12 becomes excessively thin.
Thus, the second embodiment of the present invention is configured
to execute the operation in the low-load stratified combustion
region when the engine rotation speed is relatively fast. In the
low-load stratified combustion region, the fuel stream floating in
the air forms the air-fuel mixture from the peripheral areas of the
fuel stream by mixing with the air and vaporizing the fuel directly
after the fuel is injected. Thus, the fuel stream without going
through the cavity 4 is quickly formed into a compact air-fuel
mixture directly below the fuel injection valve 11 near the spark
plug 12. Therefore, in the second embodiment of the present
invention, stable stratified combustion can be obtained regardless
of the engine rotation speed.
Moreover, the first prescribed engine load and the first prescribed
engine rotation speed are set such that the first prescribed engine
load becomes larger as the first prescribed engine rotation speed
becomes faster. When the engine load is higher, the amount of fuel
injected is larger, which sometimes results in forming an
excessively dense air-fuel mixture. Thus, by setting the first
prescribed rotation speed such that the first prescribed rotation
speed gets faster as the first prescribed engine load becomes
larger, when the engine load is high, the operation of the low-load
stratified combustion region is executed only when the engine
rotation speed is relatively fast. Thus, a danger of smoke
discharge is reduced. Moreover, when the engine load is relatively
high and the engine rotational speed is relatively slow in the
stratified combustion operating region, the operation in the
high-load stratified combustion region is executed. Since the fuel
injection amount is relatively large when the engine load is
relatively high, the mixing of the air and fuel is promoted by the
gas flow inside the cylinder to prevent forming a less dense
air-fuel mixture in the vicinity of the spark plug 12 in the
high-load stratified combustion region in which the air-fuel
mixture is formed after the fuel stream collides against the cavity
4.
However, because the total fuel injection amount increases when the
engine load is larger than the second prescribed engine load,
sufficient mixing of the fuel and air is difficult even though the
flow is intensified by faster engine rotation speed in the low-load
stratified combustion region, which increases the danger that an
excessively dense air-fuel mixture will be partially formed. Thus,
in the second embodiment of the present invention, the control unit
13 is configured to determine the direct fuel injection engine is
operating in the high-load stratified combustion region when the
engine load is higher than the second prescribed engine load.
Accordingly, when the fuel injection amount exceeds certain amount
due to increase in the engine load, smoke discharge can be
controlled and good combustion obtained by executing the operation
in the high-load stratified combustion engine in which the fuel
stream is sufficiently vaporized and mixed with the air as the
air-fuel mixture is formed after the fuel stream collides against
the cavity 4 and guided upwardly above the cavity 4.
Furthermore, when the engine load is equal to or less than the
third prescribed engine load, the fuel injection amount is small.
Therefore, there is the danger of the density of the air-fuel
mixture becoming excessively thin when attempting to ignite the
fuel after the air-fuel mixture is formed via the cavity 4 in the
high-load stratified combustion region. Accordingly, the control
unit 13 is configured to determine the direct fuel injection engine
is operating in the low-load stratified combustion region when the
engine load is lower than the third prescribed engine load. Thus,
the air-fuel mixture formed directly after the fuel is injected is
ignited thereby making it possible to prevent the combustion
stability from worsening when the engine load is low.
As described above, when the engine rotation speed becomes faster,
there is the danger that there is no sufficient time to form an
air-fuel mixture formed via the cavity 4. Even when the engine
rotation speed is relatively high and engine load is relatively
high, the stratified combustion can be obtained by igniting an
air-fuel mixture formed via the cavity 4 although the air-fuel
mixture may contain relatively dense air-fuel ratio due to the
large amount of the fuel injection. However, when the engine
rotation speed is fast with respect to time required to form an
air-fuel mixture using the cavity 4 (which is usually determined
based on the fuel injection pressure, the shape of the cavity, and
the like), there may not be sufficient time to form an air-fuel
mixture in the vicinity of the spark plug 12 via cavity 4. Thus,
the control unit 13 is configured to determine the direct fuel
injection engine is operating in the low-load stratified combustion
region when the engine rotation speed is higher than the second
prescribed engine rotation speed. Accordingly, the combustible
air-fuel mixture is positioned near the spark plug 12 even when the
engine rotation speed is fast.
Moreover, according to the second embodiment of the present
invention, the control unit 13 is configured to change at least one
of the fuel injection timing, fuel ignition timing, intake valve
closing timing and fuel injection pressure in conjunction with
switching operations among the low-load stratified combustion
region and the high-load stratified combustion region. Since these
parameters can be instantly controlled without using special
equipments, suitable operation of the direct fuel injection engine
is obtained without increasing the cost. Thus, even when a driver
of the vehicle suddenly changes the operating state of the vehicle,
the operations among the high-load and low-load stratified
combustion regions are switched without degrading the driving
performance of the vehicle.
In order to achieve good fuel efficiency and less exhaust emission
regardless of the operating state of the direct fuel injection
engine, a heat generation time after fuel ignition must be
optimized. In other words, the fuel ignition timing must be set
such that an optimum heat generation time is obtained for each
operating state. However, the fuel ignition timing is required to
be in a certain range of time span in a cycle and cannot exceed
this range. Thus, in order to accommodate this situation, the fuel
injection timing is varied to optimize the heat generation time in
the second embodiment of the present invention. Since the fuel
injection timing in the low-load stratified combustion region is
set more retarded than the fuel injection timing in the high-load
stratified combustion region, the fuel stream is allowed to pass
through the cavity 4 and sufficiently mixed after the fuel is
injected in the high-load stratified combustion region, and the
fuel stream forms an air-fuel mixture that is ignited directly
after the fuel is injected in the low-load stratified combustion
region.
In addition, in the low-load stratified combustion region, the
travel direction of the fuel stream must approach the spark plug 12
in order to reliably position the air-fuel mixture in the vicinity
of the spark plug 12. If the travel direction of the fuel stream is
preset to direct at the spark plug 12, there is a concern that
smoldering may occur on the spark plug 12 especially when the
engine load is high. Accordingly, in the present invention, the
intake valve closing timing in the low-load stratified combustion
region is set to more retarded relative to the intake valve closing
timing in the high-load stratified combustion region so that the
pressure inside the cylinder during the fuel injection is reduced
in the low-load stratified combustion region. Thus, the actual fuel
stream injection angle (injection opening angle) is slightly
increased to reliably position the combustible air-fuel mixture
close to the spark plug 12 in the low-load stratified combustion
region. Moreover, in the high-load stratified combustion region, by
closing the intake valve early, the pressure inside the cylinder is
increased and the actual fuel stream injection angle is slightly
reduced. Thus, the fuel stream is further reliably directed toward
the cavity 4 to form the air-fuel mixture inside and above the
cavity 4.
According to the second embodiment of the present invention, the
fuel injection pressure in the low-load stratified combustion
region is set lower than the fuel injection pressure in the
high-load stratified combustion region. Thus, the penetrative force
of the fuel stream is reduced in the low-load stratified combustion
region and the fuel undergoes even more vaporization because the
fuel is floating. Moreover, the air-fuel mixture distribution is
further made compact and further reliable ignition is obtained.
Furthermore, in the second embodiment of the present invention, the
interval T1 between the fuel injection timing and ignition timing
in the low-load stratified combustion region is set shorter than
the interval T2 between the fuel injection timing and ignition
timing in the high-load stratified combustion region. Accordingly,
the fuel stream is further reliably ignited while the fuel is
floating and a compact air-fuel stream formation is achieved to
prevent the exhaust emissions from worsening.
THIRD EMBODIMENT
Referring now to FIGS. 10 and 11, a direct fuel injection engine in
accordance with a third embodiment will now be explained. In view
of the similarity between the first and third embodiments, the
parts of the third embodiment that are identical to the parts of
the first embodiment will be given the same reference numerals as
the parts of the first embodiment. Moreover, the descriptions of
the parts of the third embodiment that are identical to the parts
of the first embodiment may be omitted for the sake of brevity.
Basically, the third embodiment is identical to the first
embodiment, except that an additional fuel stream is injected
during the compression stroke when the direct fuel injection engine
is operating in a relatively high-load region within the high-load
stratified combustion region. The additional fuel stream is
injected such that the additional fuel stream first hits the bottom
surface 4c of the cavity 4 and guided upwardly by the curved
surface 4b and the peripheral surface 4a.
When the fuel injection pressure is not controlled to be set higher
as the engine load becomes higher when the direct fuel injection
engine is operating in the high-load stratified combustion region,
it becomes difficult to have majority of the fuel stream collide
against the cavity inner peripheral surface 4a because the duration
of the fuel injection becomes longer as the engine load becomes
higher. In such case, the fuel stream injected during the second
half of the duration of the fuel injection would collide against
the curved surface 4b of the cavity 4. The collision angle formed
between the fuel stream and the curved surface 4b is substantially
a right angle and the fuel stream collided against the curved
surface 4b would not travel in a specific direction. Thus, the
movement of the fuel stream is not converted to a circulation
flow.
Accordingly, in the third embodiment of the present invention, when
it is determined that the majority of the fuel stream would not
collide against the cavity inner peripheral surface 4a based on the
engine load, the injection of the fuel stream is divided into two
injections as seen in FIG. 10. The first injection causes the fuel
stream to collide against the cavity inner peripheral surface 4a
creating a circulation flow in the same manner as the first
embodiment as seen in diagram (B) of FIG. 10. The second injection
causes the fuel stream to collide against the cavity bottom surface
4c as seen in diagram (C) of FIG. 10. This second injection creates
a circulation flow in the opposite direction to the first
circulation flow as seen in diagram (D) of FIG. 10. Therefore, the
first half of the fuel injection creates an air-fuel mixture in the
vicinity of the center of the cavity 4 and thereabove, while the
second half of the fuel injection creates an air-fuel mixture in
the vicinity of the inner peripheral surface 4a of the cavity 4 and
thereabove. As an overall result, one large air-fuel mixture mass
is created within the cavity 4 and thereabove. Since the two gas
flows in directions opposite from each other are created, mixing of
fuel and air is promoted resulting in excellent stratified
combustible air-fuel mixture layer. Although two fuel streams are
injected in the third embodiment of the present invention, it will
be apparent to those skilled in the art from this disclosure to
inject more than two fuel streams in a compression stroke in order
to create an air-fuel mixture optimum for combustion.
FIG. 11 illustrates the relationship between the operating regions
with respect to the engine load and engine rotation speed in
accordance with the third embodiment. As seen in FIG. 11, the
relatively high-load region within the high-load stratified
combustion is a region in which it is determined the it is
difficult to allow the majority of the fuel stream to collide
against the cavity inner peripheral surface 4a. Thus, the
relatively high-load region within the high-load stratified
combustion is regarded as a multiple fuel injection region.
According to the third embodiment of the present invention, even
when the fuel injection pressure is not controlled to increase in
response to the engine load increases in the high-load stratified
combustion region, a large air-fuel mixture mass can be created
within the cavity 4 and thereabove by injecting the fuel multiple
times during the compression stroke. Moreover, since the
circulation flows can be created in different directions by
injecting the fuel multiple times, a disturbance occurs within the
cavity 4 making it possible to promote mixing between the injected
fuel and air. Accordingly, stable combustion can be obtained while
introducing large quantities of EGR as well as combustion with good
fuel efficiency and a small amount of NOx can be obtained.
It will be apparent to those skilled in the art from this
disclosure that the multiple injections of the third embodiment can
be adapted to the direct fuel injection engine of the second
embodiment explained above. For example, the direct fuel injection
engine of the second embodiment can be configured and arranged to
execute multiple fuel injections in the region where the engine
load is higher than the second prescribed engine load so that a
large air-fuel mixture mass can be created within the cavity 4 to
obtain an excellent combustion in the relatively high-load region
within the high-load stratified combustion region.
FOURTH EMBODIMENT
Referring now to FIG. 12, a direct fuel injection engine in
accordance with a fourth embodiment will now be explained. In view
of the similarity between the first and fourth embodiments, the
parts of the fourth embodiment that are identical to the parts of
the first embodiment will be given the same reference numerals as
the parts of the first embodiment. Moreover, the descriptions of
the parts of the fourth embodiment that are identical to the parts
of the first embodiment may be omitted for the sake of brevity. The
parts of the fourth embodiment that differ from the parts of the
first embodiment will be indicated with a prime (').
Basically, the fourth embodiment of the present invention is
identical to the first embodiment, except that the spark plug 12 is
positioned further away from the fuel injection valve 11 and a
piston 3' is substituted for the piston 3 of the first embodiment.
The piston 3' of the third embodiment is basically identical to the
piton 3 of the first embodiment, except that the shape of the
cavity 4' has been modified from the cavity 4 of the first
embodiment.
When the spark plug 12 cannot be arranged close to the fuel
injection valve 11 due to limitations on the construction of the
cylinder head 2, the spark plug 12 is installed at a position away
from the fuel injection valve 11 as seen in FIG. 12. In the fourth
embodiment of the present invention, a bottom surface 4c.sub.1 of
the cavity 4' is inclined such that a part of the bottom surface
4c' including a bottom surface 4c.sub.2 that is farther from the
spark plug 12 is shallower than a part of the bottom surface 4c'
including a bottom surface 4c.sub.1 that is closer to the spark
plug 12. Moreover, the cavity 4' includes an inner peripheral
surface 4a' in which a surface 4a.sub.2 that is farther from the
spark plug 12 is less inclined toward the center axis of the piston
3' than a surface 4a.sub.1 that is closer to the spark plug 12. The
surfaces 4a.sub.1 and 4a.sub.2 are smoothly joined in
circumferential direction of the cavity 4' to form a smooth surface
of the inner peripheral surface 4a'. Thus, a stratified air-fuel
mixture layer suitable for combustion is formed in the vicinity of
the spark plug 12 by injecting the fuel stream onto the cavity
inner peripheral surface 4a' when the direct fuel injection engine
is operating in a high-load stratified state.
Accordingly, in the second embodiment, even when the spark plug 12
cannot be arranged close to the fuel injection valve 11 as in the
first embodiment, stable combustion can be obtained while
introducing large quantities of EGR when the direct fuel injection
engine is operating in the high-load stratified combustion region
as in the first embodiment. Moreover, combustion with good fuel
efficiency and a small amount of NOx can be also obtained.
It will be apparent to those skilled in the art from this
disclosure that the structure of the spark plug 12 and the cavity
4' of the fourth embodiment can be adapted to the direct fuel
injection engine of the second or third embodiment as explained
above in case the spark plug 12 cannot be arranged close to the
fuel injection valve 11.
FIFTH EMBODIMENT
Referring now to FIG. 13, a direct fuel injection engine in
accordance with a fifth embodiment will now be explained. In view
of the similarity between the first and fifth embodiments, the
parts of the fifth embodiment that are identical to the parts of
the first embodiment will be given the same reference numerals as
the parts of the first embodiment. Moreover, the descriptions of
the parts of the fifth embodiment that are identical to the parts
of the first embodiment may be omitted for the sake of brevity. The
parts of the fifth embodiment that differ from the parts of the
first embodiment will be indicated with a prime (').
Basically, the direct fuel injection engine of the fifth embodiment
is identical to the first embodiment, except that a fuel injection
valve 11' is substituted for the fuel injection valve 11 of the
first embodiment. More specifically, the direct fuel injection
engine of the fifth embodiment utilizes the fuel injection valve 1'
in cases when the fuel injection valve 11 of the first embodiment
cannot be set substantially parallel to the center axis of the
piston 3. The injection valve 11' is a multi-hole injection valve
that allows nonsymmetrical fuel injection in the axial direction of
the fuel injection valve 11'. Thus, in the fifth embodiment, the
fuel injection valve 11' is installed such that the center axis of
the fuel injection valve 11' is inclined with respect to the center
axis of the piston 3 and the fuel stream injected from the fuel
injection valve 11' forms a hollow cone shape that is substantially
symmetrical with respect to the center axis of the piston 3, as
seen in FIG. 13.
Accordingly, in the fifth embodiment, even when the fuel injection
valve 11 cannot be set substantially parallel to the center axis of
the piston 3, the shape of fuel stream is formed into a hollow cone
that is substantially symmetrical with respect to the piston 3.
Consequently, unburned HC as well as cooling loss can be reduced
when the engine is operating in the low-load stratified state as in
the first embodiment. Also, combustion with good fuel efficiency
and a small amount of exhaust gas emissions can be obtained. When
the engine is operating in the high-load stratified state, stable
combustion is obtained while introducing large quantities of EGR as
well as combustion with good fuel efficiency and a small amount of
NOx.
It will be apparent to those skilled in the art from this
disclosure that the structure of the fuel injection valve 11' of
the fifth embodiment can be adapted to the direct fuel injection
engine of the second or third embodiment as explained above in case
the fuel injection valve 11 cannot be arranged substantially
parallel to the center axis of the piston 3.
As used herein, the following directional terms "forward, rearward,
above, downward, vertical, horizontal, below and transverse" as
well as any other similar directional terms refer to those
directions of a vehicle equipped with the present invention.
Accordingly, these terms, as utilized to describe the present
invention should be interpreted relative to a vehicle equipped with
the present invention.
The term "configured" as used herein to describe a component,
section or part of a device includes hardware and/or software that
is constructed and/or programmed to carry out the desired
function.
Moreover, terms that are expressed as "means-plus function" in the
claims should include any structure that can be utilized to carry
out the function of that part of the present invention.
The terms of degree such as "substantially", "about" and
"approximately" as used herein mean a reasonable amount of
deviation of the modified term such that the end result is not
significantly changed. For example, these terms can be construed as
including a deviation of at least .+-.5% of the modified term if
this deviation would not negate the meaning of the word it
modifies.
The application claims priority to Japanese Patent Application Nos.
2003-121610 and 2003-154056. The entire disclosures of Japanese
Patent Application Nos. 2003-121610 and 2003-154056 are hereby
incorporated herein by reference.
While onely selected embodiments have been chosen to illustrate the
present invention, it will be apparent to those skilled in the art
from this disclosure that various changes and modifications can be
made herein without departing from the scope of the invention as
defined in the appended claims. Furthermore, the foregoing
descriptions of the embodiments according to the present invention
are provided for illustration only, and not for the purpose of
limiting the invention as defined by the appended claims and their
equivalents. Thus, the scope of the invention is not limited to the
disclosed embodiments.
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